Structural sandwich panels are, of course, well known building components, both in respect of conventional structures and in aircraft fabrication; the latter being a particularly preferred environment within the context of the present invention. The popularity of these panels is attributable to a relatively high strength-to-weight ratio, in turn a consequence of the now conventional construction employed; an aspect requiring no detailed elaboration as the same is very well known. Suffice it to say in summary terms, the sandwich panels of interest are comprised of a pair of relatively thin face sheets separated by and bonded or otherwise secured to an intermediate, foraminous or honeycomb member of relatively thicker dimension. The face sheets serve to distribute a load through the honeycomb to the supporting structure. Depending upon the anticipated loads to which the panel member will be subjected, all manner and variety of materials may be utilized and certain variations in the structural conformation of the honeycomb employed. Metals, polymeric resins, and impregnated fibrous materials have all been utilized in the past in this regard.
While these structural panels are very satisfactory in load transmission where the force is one applied over a rather large area, crushing resistance of the intermediate layer is not very great where the panel is subjected to point loads. Consequently, construction utilizing these sandwich panels is attended with certain complications; for even the force exerted by a fixture member such as a nail, screw or the like, passing through the panel can crush it locally or give rise to stress ultimately contributing to failure of the panel at fixture locations. Accordingly, numerous approaches have been devised over the years with an eye toward providing an effective means for securing these panels to support members. The conventional wisdom along these lines has suggested the use of a metallic insert passing through the panel member and through which itself the fastener may pass into cooperative engagement with a support such as a stud or rail. The insert isolates the compressive restraining force exerted by the fastener from the panel while it holds the same in shear during times of force application. Various patented fastener inserts are known in the prior art, to which attention is now directed.
U.S. Pat. No. 2,585,557 to Creimendahl discloses a multicomponent fastener insert for structural sandwich panels. That insert is comprised of a bushing disposed through the panel, optionally but preferably countersunk near the top to receive the head of a bolt or like fixture member. The lower portion of the bushing is provided with a reduced diameter to yield a shoulder with a fillet receiving a sleeve or collar as a separate component. When the panel is installed and the fastener tightened, the fillet is spread or flared into engagement with the face sheet of the panel immediately circumjacent the hole therethrough which receives the bushing, providing an integral mechanical linkage.
U.S. Pat. No. 2,961,760 to Horton et al is conceptually similar in its disclosure of an insert for a structural sandwich panel through which a fixture member is destined to pass, in this case preferably a rivet. The insert is, once again, basically a cylindrical sleeve or bushing sized to accept the head of the rivet, having a bore through which the shank thereof may pass. The sleeve is inserted within a hole through the sandwich and deformed in situ to establish cooperative association with the panel.
U.S. Pat. No. 3,042,156 and U.S. Pat. No. 3,355,850, to Rohe, as well U.S. Pat. No. 3,296,765 to Rohe et al disclose various types of fastener inserts for structural sandwich panels. Each of these inserts or spacers relies to one extent or another upon radial expansion of the same once disposed within an aperture through the panel in order to yield a mechanical interlock therewith.
Among other insert designs which employ a type of radial expansion or bulging of the device to anchor or otherwise provide operative interengagement between the insert and an associated sandwich panel may be mentioned those which are the subjects of U.S. Pat. No. 3,252,493 to Smith, U.S. Pat No. 3,313,079 to Phelan, and U.S. Pat. No. 3,651,563 to Volkmann. Further insight into additional background in respect of conceptually similar inserts and/or fasteners of this ilk may be gleaned from U.S. Pat. No. 2,967,593 and U.S. Pat. No. 3,008,552 to Cushman and Cushman et al, respectively, and U.S. Pat. No. 3,078,002 to Rodgers.
Another commercially-available fastener insert pertinent to the scope of the background art is one heretofore employed by the assignee of the present invention, which device is considered in much greater structural detail below. For present summary purposes, the insert is comprised of a shaft for disposition through the sandwich panel, with a chamfered tip adapted to be swaged, bent or crimped outward in a generally radial direction once the insert is positioned, and a base including a radially extending flange having a diameter greater than the through-hole in the panel. The insert is secured to the sandwich panel by adhesively bonding the flange to the mating face sheet. The shaft, while having an overall circular conformation, is provided with a reduced cross-section or undercut intermediate its length, principally for weight savings. That shaft includes a stepped bore for receiving a fastener, comprised of a tapered area proximate the tip for cooperation with the head of the fastener in order that the same may remain flush with the surface of the panel.
The commercial device immediately aforesaid offers a great many advantages over the patented prior art devices of the preceding discussion. Structurally, the same is of considerably less complexity and offers the further advantage of unitary construction as compared with most of the previous approaches. This contributes, in turn, to an ease of installation while maintaining the desired objectives of good pullout resistance and force transfer at fastening points. However, even this device suffers certain indigenous problems. Perhaps most significant as respects the present invention is the fact that this device is one produced by machining techniques, therefore limiting dimensional features (such as recess depth for the fastener seating tip portion), thickness of the restraining flange and even wall thickness due to inherent limitations in the machining process itself. Another significant commercial problem is the cost associated with machining, taking into account the capital expenditure for equipment, scrap generated during the procedure, disposal of that scrap and like considerations. Looking more specifically to production costs, where the panels are made of fiberglass or similar materials, aluminum inserts are satisfactory and can be produced at about 23 cents each; but, where graphite composites are employed aluminum may not be used, requiring titanium inserts at a cost of about $9.00 each. When one then considers the staggering number of inserts utilized each year, astounding costs can mount.
Accordingly, a need exists for a fastener insert which retains the basic simplicity of unitary design and its attendant advantages, but which improves upon the structural characteristics of the insert while substantially reducing cost of production.